New study favors violent mash-up Moon hypothesis

A new study that analyzed isotopic differences in potassium contained in terrestrial and lunar rock samples is helping to shed light on the formation process that led to the creation of Earth's Moon. The research could help reveal the nature of a cataclysmic collision between Earth and a Mars-sized body, which is believed to have taken place in the distant past.

Elements in rocks that originated in
different parts of the solar system are known to boast distinctive – albeit minute – differences in the compositions of their isotopes. By observing
this sort of isotopic fingerprint, scientists
are capable of determining where a sample of material from outer
space originated.

Researchers have
attempted to apply this technique to elements in lunar rocks, including tungsten and potassium, in order to gain
insights regarding the formation process that sculpted Earth's Moon.
The earliest isotopic analysis carried out by researchers discovered
that many elements in Moon rocks bore a nearly identical isotopic
fingerprint to their terrestrial cousins.

The revelation
that the Moon appeared to be created largely from the same material
as Earth's mantle led scientists to abandon the previously held
hypothesis that Earth's companion had formed as a result of a grazing
collision between the proto-Earth and a Mars-sized body, nicknamed
Thea.

Numerical
simulations of this scenario suggest that isotopes found in elements
in terrestrial and lunar samples would differ significantly, as
around 60 – 80 percent of the satellite's material would belong to
the impactor, which originated in a different part of the Solar System.

Since the isotopes
found in lunar and terrestrial samples were nearly identical, this
could not be the case. Scientists were then tasked with developing
new hypotheses that would explain the creation of our Moon
predominantly from the material of the proto-Earth.

Whilst many
hypotheses were proposed in the aftermath of the discovery, two in
particular rose to prominence. The first Moon-creation model suggests
that the impactor struck proto-Earth in a relatively low-energy
collision, melting a part of our young planet's mantle, and flinging
it out into orbit like water escaping a spinning ball. This created a
disk of molten debris.

Illustration of the two leading formation models for Earth's Moon(Credit: Kun Wang)

The team behind
the hypothesis proposed that, as a by-product of the collision, both
proto-Earth and the impactor would be enveloped in a silicate vapor
atmosphere, which would act to facilitate the transfer of molten
material to the impactor. Whilst the model could theoretically
account for the isotopic similarities between terrestrial and lunar
rocks, the transfer of magma debris to the proto-Moon would be
incredibly slow – too slow for the mixed material to fall back to Earth according to Kun Wang, one of the authors of the study.

The
second hypothesis is predicated on a high-impact collision between
proto-Earth and Thea. Under this model, the force of the impact would
have vaporized both the impactor and the majority of Earth's mantle,
creating a vast atmosphere composed of supercritical fluid that extended across a region of space 500 times the volume of
present day Earth. Over time, Earth's Moon would have formed out of
the superfluid atmosphere. This model would allow for a greater
mixing of materials from the two celestial bodies in a shorter time
frame.

The
new study observed differences between potassium isotopes present in
samples of moon and terrestrial rock. The seven lunar rocks analyzed
by the team were returned to Earth over the course of a number of
Apollo-era missions. These rocks were compared with eight terrestrial
samples that were deemed to be representative of Earth's geochemical
make up.

Wang, a geochemist at Washington University, St.
Louis, along with study co-author Stein Jacobsen, a professor of geochemistry at Harvard
University, Massachusetts, leveraged a method for isotopic analysis that, developed
by the two in 2015, boasts a precision 10 times greater than any
previous technique.

The team
discovered that the lunar rocks contained more of the heavy isotope
potassium-4 than their Earthly cousins. If the Moon had formed as per
the silicate atmosphere model, exactly the opposite would have been
predicted. Therefore the findings support the hypothesis that the Moon
was created in a catastrophic collision, which vaporized much of
Earth's mantle.

So, next time you
see the Moon, try and look past its placid beauty to the incredible
violence of its past.